This week's psychology lesson: folie a deux.

Other variants of this RARE disorder include: folie a trois, folie a quatre, folie a famille, and folie a plusieurs. Also known as, "shared psychotic disorder", the original name literally means madness for two (or three, or four, or a family, or many), and its primary symptom is a delusion that is shared by 2 or more people who live together. In other words, this disorder is characterized by contagious delusions.

Delusions are tricky. They're defined as firmly-held beliefs that remain present despite significant and compelling evidence to the contrary. Despite what some may argue, the psychological/psychiatric community has decided that commonly-held religious beliefs may NOT be considered delusions. Delusions can be bizarre (a machine in Texas controls my left eyeball and my thoughts) or non-bizarre (my neighbors are stealing my clothes). They are associated with a number of disorders, including schizophrenia, dementia, and substance withdrawal (among others). In the case of Folie a Deux, the delusions are typically not bizarre.

Two examples:

(1) Harriet begins to suspect that someone is stealing her shoes, wearing down the soles, and then giving them back. Her husband, Hank, also comes to believe this and starts hiding his shoes before he goes to bed at night.

(2) Hal believes his food is being poisoned by the government. His brother Hugo stops eating take-out because he starts to believe it too.